Hermetically sealed terminals have been widely used for many years in refrigeration equipment, wherein an electrical connection is made with component mounted within a sealed receptacle. In the case of refrigeration headers, the motor-compressor units are hermetically sealed and are normally located outside the building they are intended to heat or cool. Normally these units are very reliable and operate without difficulty over many years of service and, in general, are quite safe when installed, serviced and inspected by trained professional personnel. Even under these conditions, however, there is an occasional dangerous explosive venting of the motor-compressor receptacle through the electric terminal, the terminal seals being arced-away or shattered by explosive forces generated within the sealed system. If the explosive venting occurs when no one is close to the system, the resulting fumes, odor and/or flames, which are rapidly propelled through the openings in the terminal body, would be of concern, but not necessarily injurious. This is not always the case, however, and numerous injuries have resulted from ruptured terminals.
While various safety devices have hitherto been utilized to safeguard against dangerous venting of the refrigeration gases and oil, including the provision of thermal protectors which will shut down the motor-compressor unit if a predetermined internal circuit temperature is reached, none of these devices has proven to be wholly satisfactory. For example, a locked rotor will cause currents of approximately 5-6 times the rated load current of the unit to flow through the terminals to the motor winding. Because it is necessary for the external branch circuit fuses or circuit breakers to have a considerable time delay before they interrupt the current to the compressor (which delay is required for normal starting), these fuses and breakers do not rapidly trip to shut off the current to the unit. As a further complication, service personnel and homeowners sometimes attempt to restart the motor-compressor unit by installing slow-blow fuses of much higher amperage than recommended, or use no fuses at all--just copper bars or alligator clips for supplying current directly to the unit.
If during attempted startup the motor-compressor unit is in a major fault condition, any of the foregoing conditions could result in very dangerous explosive venting of the contents of the sealed unit and at the same time provide considerable electric power for ignition of the venting materials.
Various other expedients have been proposed to overcome the problem, but none has been totally satisfactory. For example, various types of temperature responsive circuit interrupters have been proposed for incorporation in the terminal itself. One such device utilizes thermally sensitive pellets which act to release a spring-bias contact as the pellet melts. While effective, the current interrupting action is too slow in operation due to the relatively large mass of material which must be melted and caused to flow before the gap between the conductive members is at the maximum. While efforts have been made to enhance the sensitivity of such devices and provide fast-tripping action, such efforts have not been totally successful in shutting down the motor-compressor before blow-outs occur.
In contrast to the foregoing, the aforementioned copending application Ser. No. 07/235,027 provides a terminal construction which provides for substantially instantaneous cut-off of current when a predetermined current induced temperature level is reached, including a unique pin construction whereby Portions of the terminal pins are physically separated from the remainder of the terminals while maintaining the integrity of the hermetic seals.
An analysis of past failures of hermetically sealed terminals leads to the conclusion that undesirable venting or blow-out of the terminals occurs because of several factors:
(1) Very high currents passing through the terminal due to major shorts or contamination. PA1 (2) The high currents and/or contamination cause very high temperatures and arcing over the internal surfaces of the terminal. PA1 (3) In the case of terminals utilizing glass-to-metal seals, the very high current induced temperatures cause the glass seals to become more conductive and thereby accelerate the temperature build up until the physical strength and integrity of the seal is explosively lost. PA1 (4) In the case of so-called "soft-seal" terminals wherein the terminal pins are sealed using various distortable dielectrical materials, the seal is gradually arced away until failure results.
In order to prevent these potentially catastrophic failures, the subject application teaches a terminal construction wherein the terminal pins are formed by inner and outer pin parts interconnected by a high-temperature, high-current fuse link formed from a silver alloy which will melt and/or vaporize when heated to a predetermined temperature, thereby rapidly breaking the electrical connection between the inner and outer pin parts. Preferably, the fuse link are formed from a silver alloy having a melting point in the range of 1100.degree.-1450.degree. F., although the melting point and current carrying capacity of the fuse link will be chosen in accordance with the electrical characteristics of the device being operated. For example, in the case of a refrigeration header, the fuse link will be sized to carry current equal to the locked rotor amperage of the compressor for a continuous time period in excess of three minutes and a current of two times the locked rotor amperage for approximately two to six seconds
The fuse link and the adjacent ends of the inner and outer terminal pins are surrounded by a protective capsule which provides an air space or cavity surrounding the fuse link, the cavity providing space for the melted or vaporized fuse link material to migrate, thereby assuring clean separation of the pin parts. In cases where a major fault occurs, as where very high short circuit currents are encountered, the vaporization of the fuse material is extremely rapid and very high internal capsule pressures will result. To assure rapid current cut-off, the capsule is designed to be popped apart and separated, thereby physically separating one of the pin parts of the terminal pin from the other pin part and the body of the terminal, such separation of the parts serving to prevent continuous arcing and to exhaust the built-up capsule pressure, thereby cutting off current flow without destroying the integrity of the hermetic seal.
In accordance with the aforesaid invention, either the inner or the outer pin part may be separated from the remainder of the terminal, although preferably the terminal will be constructed so that the outer pin part will separate, thereby giving a visual indication that there has been a major fault condition. To ensure the desired separation of the pin parts, the capsule is preferably formed in two parts, the first comprising a cup-like body adapted to receive one of the pin parts, the body being closed by a cap adapted to make snap-lock connection with the body, the cap engaging the other pin part. When pressure builds up in the capsule due to fuse vaporization, the path of least resistance is through the cap and the areas of the sealing member surrounding the cap, which effectively blows-away the cap, the adjacent pin part and the surrounding areas of the sealing member.
The terminal construction described above may be utilized with either "soft-seal" terminals wherein the terminal pins are mounted to the terminal body by means of a dielectric material which is molded in situ to the body and pins, or utilizing "hard seal" techniques wherein the pins are mounted in the terminal body by means of glass-to-metal or ceramic-to-metal seals. Where "hard seals" are utilized, at least one side of the terminal body, including the fuse links and the protective capsules, will be encased in a molded in situ dielectric sealing material.
The present invention relates to a modification of the foregoing terminal construction wherein, while permitting release of the pressures generated when the fuse link is vaporized, the integrity of the terminal will be maintained while at the same time the terminal is rendered inoperative and incapable of reuse.